In batch mixing, a batch of a liquid or of a liquid suspension is loaded into a mixing tank and means for introducing a gas into the liquid is provided. An impeller-type agitator provides agitation of the liquid or of the liquid suspension while the gas to be reacted with the liquid suspension is introduced, usually by nozzles or so-called sparging rings in the form of gas bubbles. The mixing process proceeds until a selected process indicator indicates a desired degree of completion of a chemical reaction between the liquid suspension and the gas, whereupon the mixing process is terminated and the thus reacted product is emptied from the mixing tank. Numerous batch mixing systems are known in which improved mixing effectiveness has been sought by introducing a number of particular mixing system designs, tailored to achieve particular results. A measure of the effectiveness of the mixing process in a batch mixing system is a quantity referred to as a "mass transfer coefficient," which provides an indication of the effectiveness of mass transfer during the mixing process. The literature on batch mixing systems is replete with examples of improved designs for an agitator, impeller, or propeller, the incorporation of baffles disposed on the inside wall of a mixing tank, the introduction of draft tubes functioning as shrouds, and various gas introduction means and their location with respect to the impeller within the mixing tank. In some mixing system designs a stator having stationary blades or vanes is used in proximity to the impeller so as to achieve improved mixing characteristics for certain materials or under certain conditions.
Another design variable disclosed in the literature is the mixing tank itself and, more particularly, the design of the tank bottom of a mixing system. As viewed from the inside of a mixing tank, tank bottom may be a flat bottom or it may be a concave bottom, or as is the case in certain pressure vessels, it may be a convex bottom. Thus, with respect to the contour of the bottoms of mixing tanks, the disclosures known to the applicant can be accurately classified as follows:
Mixing systems having a flat bottom are disclosed in the following U.S. Pat. Nos.: 4,454,078 (Jun. 12, 1984); 4,190,371 (Feb. 26, 1980); 4,207,275 (Jun. 10, 1980); 3,875,057 (Apr. 1, 1975); 2,433,592 (Dec. 30, 1947); and 1,255,944 (Feb. 12, 1918).
Contoured tank bottoms or chamber bottoms are disclosed in the following patents: U.S. Pat. No. 4,054,519 (Oct. 18, 1977) discloses a hydraulic attrition unit for a marine toilet, showing in a portion a FIG. 3A thereof, a hemi-toroidal chamber bottom. In U.S. Pat. No. 3,488,038 (Jan. 6, 1970) there is disclosed a stirrer, shown in a FIG. 1 as having a propeller (18) directing liquid flow downwardly toward a contoured and rotating flow guiding body (19), whereupon the liquid is shown as flowing along a flat bottom portion of a container (11), and past suppressor vanes (20) attached to side walls of the container. U.S. Pat. No. 2,521,396 (Sep. 5, 1950) discloses a gas and liquid contact apparatus, showing in a FIG. 1 thereof a tank (1) having a uniformly concave tank bottom, an impeller (9) and a rotary gas target (11) attached to a shaft 10). The rotary gas target or gas distribution system directs a gas upwardly toward the impeller. The impeller ( 9) directs a liquid flow upwardly. In U.S. Pat. No. 2,123,496 (Jul. 12, 1938) there is disclosed drink mixer having a toroidally shaped bottom of a receptacle (12), as depicted in a FIG. 1 thereof. In U.S. Pat. No. 2,016,647 (Oct. 8, 1935) there is disclosed a device for treating paper stock, FIGS. 1-3 thereof showing in these embodiments various contoured bottoms of a receptacle (10), and indicating tubular members or shrouds having various configurations (15, 15a, 15b, and 15c). In U.S. Pat. No. 1,960,613 (May 29, 1934) there is disclosed a method for bleaching paper pulp, and showing in FIGS. 1 and 2 thereof a tank (1) having a hemi-toroidal tank bottom delineated by a central ridge (9), an axial flow impeller (3), and a degrader (2) in which the pulp is given a downward movement (FIG. 1) by the impeller and alternatively an upward movement (FIG. 2).
Another publication, titled The Attainment of Homogeneous Suspension in a Continuous Stirred Tank, by S. Aeschbach and J. R. Bourne, The Chemical Engineering Journal, Elsevier Sequoia S. A., Lausanne (1972) describes results of a comprehensive study of various configurations of tanks and positions of an impellets immersed into a suspension of PMMA particles in a hydrocarbon liquid. This publication does not address the question of mixing, rather it attempts to address the particle size distribution in the liquid before and after stirring of the liquid suspension by impellers. In that publication, on pages 238 and 239 thereof, there are shown as a "case 6" and a "case 7" a system within a tank having a hemi-toroidal tank bottom and having baffles in the form of a draft tube. The propeller directs the flow of the liquid suspension upwardly. The particle size distribution curves shown in FIGS. 6 and 7, respectively, indicate that a more homogeneous suspension is retained after a period of agitation of the suspension by using the configurations and conditions described as "case 6" and "case 7", respectively. The conclusions of that publication, on page 242 thereof, indicate that conventional flat-bottomed propeller agitated tanks were not producing homogeneous suspensions, while the empirically determined contoured tank bottom provided more homogeneous suspensions in terms of the measured particle size distribution. It is also pointed out in the conclusions section that constant propeller tip speed was seen as the relevant criterion to provide homogeneous suspensions in a scaled up version of the stirrer apparatus having the contoured bottom.
It is apparent from a detailed review of the foregoing publications, and as related to batch mixing systems, that at least the following major aspects or features of a batch mixing system designed for mixing a gas with a liquid suspension have either not been considered or have not been appreciated in the prior art with respect to an influence of any one aspect or feature or of a combination of aspects or features on a measure of mixing effectiveness achievable in a batch mixing system:
(i) the influence of swirl and turbulence of the liquid flow as induced by an impeller disposed in a mixing tank; PA1 (ii) the influence of the contour or profile of the bottom of a mixing tank in the case of a "down-flow" batch mixing system, i.e., a batch mixing system in which an impeller provides a flow of a liquid suspension which is initially directed downwardly toward the tank bottom; and PA1 (iii) the influence of the disposition within a mixing tank of an assembly including at least an impeller, a stator adjacent to the impeller, a shroud adjacent to a radial periphery of both the impeller and the stator, and a gas sparging means radially coextensive with the shroud.
Accordingly, it is desirable to provide a batch mixing system having an improved mixing effectiveness for mixing a gas with a liquid suspension. Such an improved mixing system can be advantageously used in the fields of fermentation, aeration, and generally in mixing applications using reactive gases to promote controlled chemical reactions in liquid suspensions.